Screw drive actuators are known in the prior art which convert rotary motion into linear motion. In one type of prior art screw drive actuator, a rotating nut member cooperates with a corresponding screw member to produce the desired linear movement. A plurality of longitudinally spaced separate grooves running around the inside of the nut member and a plurality of balls held in place by a retainer member interposed between the nut and screw members, cooperate with the screw threads to convert the rotary motion of the nut into linear motion of the screw member. While this prior art actuator has been satisfactory in many respects, it does not provide for an overload mechanism whereby the nut member continues to rotate even if the screw member jams nor does it permit manual movement of the threaded rod.
Another prior art screw drive actuator provides an assembly of three rollers spaced around and engaging a member which is to be longitudinally driven. The rollers have a garter type spring surrounding them so as to urge them radially inwardly against the longitudinally driven member. The roller assembly is rotated around the longitudinal member so as to produce the desired motion conversion. While this device does provide overload function in that the rollers are allowed to move radially outwardly, this device suffers from the drawback that it is relatively complicated and time consuming to assemble.